The present invention relates to an improved voltage-controlled-oscillator with a modulator (which will be hereinafter referred to as a VCO) which may provide constant modulation sensitivity characteristics with respect to an oscillation frequency control voltage (which will be hereinafter referred to as a control voltage) in a wide range.
A conventional VCO is generally used in a car telephone and a portable transmitter, etc. requiring a small size, in which an oscillation frequency may be adjusted by a control voltage, and also may be directly modulated to generate an output. FIG. 2 is a circuit diagram of a first example of the VCO in the prior art. Referring to FIG. 2, power is applied from a power supply terminal 20 through an A.C. blocking coil 1 to a collector of an oscillation transistor 7. Bias resistors 3, 4 and 5 are interposed between the collector of the oscillation transistor 7 and a base thereof, between the base and the ground, and between an emitter of the transistor 7 and the ground, respectively. An earthing capacitor 2 is interposed between the base of the transistor 7 and the ground. Feedback capacitors 6 and 8 are interposed between the emitter of the transistor 7 and the ground and between the collector and the emitter, respectively. Further, to the collector is connected a resonance circuit formed by resonance capacitors 9 and 13, variable capacitance diodes 10 and 14 and a microstrip line 18 formed on a substrate, thereby constituting a base grounded type modified Colpitts oscillator circuit. To a cathode of the variable capacitance diode 10 for modulation is applied a modulation signal supplied from a modulation signal input terminal 23 and suitably voltage-divided by modulation signal voltage-dividing resistors 15 and 17. Further, to a cathode of the variable capacitance diode 14 for oscillation frequency control is applied a control voltage supplied from a control voltage terminal 21 through an A.C. blocking coil 12. An oscillation output terminal 22 is connected through an output coupling capacitor 19 to the microstrip line 18. Reference numeral 11 designates a bypass capacitor.
In the above-mentioned circuit, capacitance of the variable capacitance diode 14 is controlled by the control voltage applied to the control voltage terminal 21 to regulate a resonance frequency of the resonance circuit. Further, capacitance of the variable capacitance diode 10 is controlled by the modulation signal applied to the modulation signal input terminal 23 to modulate the resonance frequency, thereby generating a frequency modulated wave from the oscillation output terminal.
FIG. 3 is a circuit diagram of a second example of a VCO in the prior art, in which the same circuit devices as in FIG. 2 are designated by the same reference numerals with explanation therefor omitted.
Referring to FIG. 3, in the resonance circuit connected to the collector of the oscillation transistor 7, the control voltage is applied to both the cathodes of the variable capacitance diodes 10 and 14 for modulation and oscillation frequency control, while the modulation signal is applied to an anode of the variable capacitance diode 10. Reference numerals 24 and 25 designate a resistor and a resonance capacitor.
In the above-mentioned circuit, capacitance of the variable capacitance diode 14 is controlled by the control voltage, and capacitance of the variable capacitance diode 10 is controlled by a potential difference between the control voltage and the modulation signal.
Recently, subscribers for car telephones have increased to result in an increase in the number of channels, requiring a wide bandwidth. Therefore, it is desirable to widen an oscillation frequency range in the VCO and provide uniform modulation sensitivity characteristics over the bandwidth of the oscillation frequency.
Referring to FIG. 4, a curve A shows control voltage-capacitance characteristics of the variable capacitance diode 14, wherein the higher the control voltage, the smaller a change rate of capacitance. Conversely, the lower the control voltage, the larger the change rate of capacitance. In FIG. 5, a curve A shows control voltage - oscillation frequency characteristics of the VCO shown in FIG. 2, in which a change rate of frequency is small in the range of high control voltage, and conversely, it is rapidly increased in the range of low control voltage. Therefore, in the conventional VCO shown in FIG. 2, a change in capacitance of the variable capacitance diode 14 by the modulation signal in the range of high control voltage is widely different from that in the range of low control voltage. As a result, control voltage-modulation sensitivity characteristics are rendered ununiform as shown by a graph A in FIG. 6, wherein the higher the control voltage, the lower the modulation sensitivity, and conversely, the lower the control voltage, the higher the modulation sensitivity.
The conventional VCO shown in FIG. 3 improves the VCO shown in FIG. 2 in such a point that the modulation signal is applied through the resistor 24 to the variable capacitance diode 14 for oscillation frequency control, thereby obtaining a reduced change in capacitance of the variable capacitance diode 14 by the modulation signal as compared with the VCO in FIG. 2. A graph B in FIG. 6 shows the control voltage - modulation sensitivity characteristics of the VCO in FIG. 3, wherein changes in the modulation sensitivity in the range of high and low control voltages are improved, but not yet satisfactory.
Additionally, in the frequency synthesizer using the conventional VCO having non-linear control voltage-oscillation frequency characteristics as mentioned above, there occurs a problem such that noise characteristics are deteriorated in the range of low control voltage and large change rate of oscillation frequency.